In optical transceivers, it is desirable to integrate as many photonic components in one chip as possible. As integration density increases and the sizes of photonic components shrink, however, it is increasingly difficult to integrate a photonic integrated circuit (PIC) with other optical components such as fibers because the mode size in the waveguide of the PIC shrinks correspondingly. For example, the mode size of a typical 450 nanometer (nm)×220 nm waveguide in a silicon-based PIC is roughly the size of the waveguide itself, but the mode size of a standard single-mode fiber is as large as 9.2 micrometers (μm). It may be necessary to transfer the optical modes from a high-index-contrast well-confined waveguide (e.g., a PIC) to a low-index-contrast large waveguide (e.g., an optical fiber). However, such transfer may cause undesirable optical losses, which should be minimized.
Existing methods fabricate an edge coupling device by tapering down a width of a high-index-contrast waveguide in order to transfer a mode from the high-index-contrast waveguide to a large, low-index-contrast waveguide. However, optical loss due to the transition may depend on a minimum width of the high-index-contrast waveguide, and a small minimum width of the taper (e.g., less than 80 nm) must be fabricated in order to achieve acceptable optical loss. Other methods fabricate an edge coupling device by transferring the optical mode from a thick, high-index-contrast waveguide to a thin slab, high-index-contrast waveguide made of silicon. The thin slab itself is used as the edge coupler. However, the mode size using the thin slab, high-index-contrast waveguide is still limited. Other existing methods are also insufficient for transferring optical modes from a high-index waveguide to a low-index waveguide. For example, a simple inverse taper may have a limited mode size and a trident edge coupling device may not achieve a low coupling loss.